DESCRIPTION (applicant's abstract): This study will examine possible circadian and sleep/wake homeostatic processes underlying adolescent sleep delay through four proposed experiments. Study 1 will test the hypothesis that intrinsic circadian period is longer in mid/post pubertal adolescents than pre/early pubertal adolescents by using a 28-hour day forced desynchrony protocol; intrinsic circadian period will be derived from regression through successive dim-light salivary melatonin onset and offset phases. This study will also examine the influence of adolescent maturation on the sleep/wake homeostatic system by making an assessment of the sleep recovery process following sleep deprivation, and evaluating the wake-dependent influence on sleep tendency. Circadian phase will be controlled. Study 2 will examine two hypotheses related to mid/post pubertal adolescents: that 1) they are more sensitive to evening light than morning light and 2) they have a greater sensitivity to evening light than younger adolescents and young adults. Both hypotheses will be tested by examining the suppression of melatonin secretion in the presence of lights with graded intensity (from < 1 lux, control, to 1,500 lux). Study 3 will evaluate the hypothesis that there is a phase delaying property of evening light which affects more mature adolescents stronger than less mature adolescents. Both groups will be exposed to the same bright light stimulus: the delay of the output of the circadian oscillator will be assessed. Study 4 will examine the notion that a sleep deficit differentially affects the balance of circadian and homeostatic regulation of the timing in a variety of behaviors. Assessment will be made by repeating the forced desynchrony protocol in Study 1, with conditions that permit less sleep. On the basis of these hypotheses, explanations for developmental changes in the bioregulatory processes affecting sleep may be elucidated, with the hope that more complex adolescent behaviors may eventually be understood.